Many epoxy adhesives designed for ambient and heat cure applications employ a polyamide as the curing agent, either alone or in some cases in combination with other curing agents. Polyamides comprise the reaction products of dimerized fatty acid (dimer acid) and polyethyleneamines, and usually but optionally, a monomeric fatty acid. Dimer acid is prepared by the oligomerization of certain monomeric fatty acids, usually tall oil fatty acid (TOFA), though sometimes other vegetable acids are substituted. Commercial products generally consist of mostly (&gt;70%) dimeric species, with the rest consisting mostly of trimers and higher oligomers, along with small amounts (generally less than 5%) of monomeric fatty acids. Any of the higher polyethyleneamines can be employed in the preparation of polyamides, such as diethylenetriamine (DETA), triethylenetetramine (TETA), tetraethylenepentamine (TEPA), or pentaethylenehexamine (PEHA), though in actual commercial practice the polyethyleneamine most commonly employed is TETA.
Polyamides are employed because they allow for the formulation of adhesives with excellent flexibility, adhesion and water resistance, most likely due to the hydrophobicity imparted by the fatty nature of the starting materials. Nevertheless, there are several properties of polyamide curing agents that would benefit from improvement.
Polyamide curing agents suitable for adhesive applications have relatively high viscosity. There has been a need to reduce the viscosity of the epoxy resin binder employed in adhesives for a number of reasons. Primary reasons include ease of application and an ability to increase the amount of the fillers used in adhesive formulations. Attempts made to lower the viscosity of polyamide curing agents include employing low viscosity polyamine curing agents as well as low viscosity plasticizers and modifiers. However, these modifications adversely affect the properties of adhesives formulations which employ such materials.
U.S. Pat. No. 2,450,940 and U.S. Pat. No. 2,705,223 both describe the preparation of polyamide resins useful for curing epoxy resins by the condensation of dimerized or polymerized fatty acids with polyethyleneamines such as ethylenediamine (EDA) and DETA.
U.S. Pat. No. 5,021,482 describes the preparation of polyamides from polymerized fatty acid and a mixture of amines comprising a polyalkylene polyamine and an N-aminoalkylpiperazine, preferably N-aminoethylpiperazine (AEP). The polyamides are utilized as adhesion promoters for PVC plastisols. Because high amine content in such an adhesion promoter destroys the acid catalysts employed in top coats applied to such plastisols, this invention is directed toward the preparation of polyamides with an amine value less than about 225. In order to achieve these low amine values, the percent by weight of amines utilized is less than 30%, preferably less than 25%. Although no viscosity of the neat polyamides prepared in '482 is reported, the viscosity of the product of example 1 is 8,880 cP at 25.degree. C., at a calculated solids of only 50%, assuming that 1 mole of water is lost for every 295 g of polymerized fatty acid in the composition. Thus, these products are very high in viscosity, and of little value in modem coatings applications, where environmental regulations require that only limited amounts of solvent can be utilized in the final coating formulation.
CS 266519 discloses an extremely broad range of polyamide resins prepared by condensing carboxylic acids (avg mol wt 146-650) with polyamines composed of 20-90% aliphatic polyamines H.sub.2 N(CH.sub.2 CH.sub.2 NHCH.sub.2 CH.sub.2).sub.1-4 NH.sub.2 and 10-80% heterocyclic polyamines H.sub.2 N(CH.sub.2 CH.sub.2 NH).sub.0-4 CH.sub.2 CH.sub.2 Z(CH.sub.2 CH.sub.2 N).sub.0-4 H.